The invention relates to a method for accurately determining the distance d of an object with respect to and by means of an FM-CW radar comprising a VCO generating a high frequency signal F having an instantaneous frequency f whose frequency is linearly modulated by a modulator GDS. The modulator generates a sawtooth supply voltage DDS, both having a duration T during which the signal F describes a linear frequency excursion of a predetermined, constant, accurately known value .DELTA.F. A transmitting-receiving aerial and a mixer receive the transmitted signal F and a received signal F' having an instantaneous frequency f' after reflection from the object whose distance d is to be measured. The mixer produces a signal Fb having a frequency f.sub.b.
In accordance with one embodiment of the invention the duration T of each sawtooth voltage (output of GDS) and the frequency (output of VCO) is predetermined, constant and known accurately.
In accordance with a further embodiment the radar of the FM-CW type comprises a control loop arranged between the output of the mixer and the input of the modulator GDS for keeping the frequency f.sub.b of the mixer output signal Fb substantially constant.
The invention also relates to a range measuring apparatus for putting into effect the method covering the above-described embodiments.
In the case in which the radar comprises a control loop it is of a type such as disclosed in, for example, Applicant's French Patent Specification No. 1,557,670 corresponding to U.S. Pat. No. 3,588,899.
The invention is preferably used for measuring comparatively short distances. Assuming the values of the parameters f.sub.1 (centre frequency of the signal F) and .DELTA.F of the radar to be fixed it is possible to define a small range of maximum distances, for example from d.sub.o to 1.1 d.sub.o, for which the precision obtained in measuring d is substantially the same as the precision with which it is possible to measure f, for example 10.sup.-4. Outside this range the invention can no longer be used and the precision to decreases to 10.sup.-3 and beyond. For values of distances shorter than d.sub.o the absolute value in the measurement of d remains constant, that is to say the relative error increases when d decreases, becoming equal for example at 10.sup.-3 for d=0.1 d.sub.o. Consequently the invention appears to be advantageous for a distance range covering approximately an order of magnitude from 1 to 10 meters, or from 10 to 100 meters, for example.
In a preferred embodiment, the invention may be applied to measuring the level of a liquid, of hydrogen or liquid oxygen, for example in containers which are a few meters high, with an absolute error which is approximately equal to 1 mm.
The basic equation from which the distance can be measured when a FM-CW radar of the above-described radar is used and which is generally used as a radio altimeter is written: EQU f.sub.b /.tau.=.DELTA.F/T (1)
in which formula .tau. is the period of time required by the wave for travelling the distance 2d between the radar and the object and back to the radar after reflection from the object, that is to say: EQU .tau.=2d/c (2)
c being the speed of propagation of the electromagnetic wave.
From the formulae (1) and (2) it is possible to derive the value of d as a function of the values measured for f.sub.b, F and T: EQU d=(cf.sub.b /2.DELTA.F)T (3)
When the range measuring apparatus is the apparatus described in French Patent Specification No. 1,557,670, the parameters f.sub.b and .DELTA.F are maintained at substantially constant values and the distance d is proportional to the duration T of a sawtooth. The precision obtained in measuring d is then equal in a first approximation to the sum of the precisions obtained for f.sub.b, .DELTA.F and T, that is to say an error on the order of .DELTA.d/d=10.sup.-2, while the relative error of f.sub.b, .DELTA.F and T are of the order of 10.sup.-3, which is already difficult to obtain in the prior art. It is also conceivable that a wave is transmitted whose frequency varies linearly in the form of a sawtooth having a frequency excursion .DELTA.F and a duration T, which are kept constant. In the latter case, .DELTA.F and T become parameters, and f.sub.b becomes the variable as a function of the distance d, in formula (3), the precision of d remaining the same as described above for the case where T is the variable. It should be noted that in the two cases considered above (T variable or f.sub.b variable as a function of d), the value of f.sub.b, during the time T in which the excursions of a sawtooth occur, may be considered as being constant, its variation being many orders of magnitude smaller than the smallest errors considered in the present text. This results from the fact that the relative speed between the object and the radar, for the considered applications, is very low, compared to the repetition rate of the sawtooths.